1. Field of the Invention
The present invention relates to a method of speed stage shifting of an automatic transmission in a vehicle such as an automobile.
2. Description of the Prior Art
In the automatic transmission in a vehicle such as an automobile which generally comprises a housing and a speed change gear mechanism including a plurality of rotary gear members and a plurality of friction engaging means such as clutches and brakes adapted to be selectively engaged and disengaged for selectively interconnecting said rotary gear members and said housing so as to selectively provide a plurality of speed stages, it is important that, in shifting the speed stages among said plurality of speed stages according to running conditions of the vehicle, engagement or disengagement of the friction engaging means is gradually proceeded at a critical moment when the friction engaging means is put into or out of a substantial engagement, in order to accomplish the speed stage shifting smoothly with no abrupt change of torque being induced in the speed change gear mechanism or in the housing of the automatic transmission which supports the speed change gear mechanism with reaction.
Conventionally, such a gradual engagement or disengagement of the friction engaging means at the critical moment is generally obtained by holding the hydraulic pressure which is being supplied to or exhausted from one of the friction engaging means to be engaged or disengaged at an intermediate level for a certain period so that in the meantime a sliding engagement is available in the friction engaging means. When one friction engaging means is disengaged while another friction engaging means is engaged for the speed stage shifting, the exhaust of the hydraulic pressure from said one friction engaging means and the supply of the hydraulic pressure to said other friction engaging means are generally so controlled that the temporal holding of the hydraulic pressure at an intermediate level in the exhaust process overlaps the temporal holding of the hydraulic pressure at an intermediate level in the supply process. Such an art is shown in, for example, U.S. Pat. No. 4,722,247, although the art itself is conventional and not the subject of said U.S. patent.
However, such a control of the hydraulic pressure requires a delicate control operation of hydraulic pressure control means including valves for controlling supply or exhaust of the hydraulic fluid to or from the friction engaging means, valves for controlling the flow of the hydraulic fluid bypassing the friction engaging means, fluid pressure accumulators, back pressure control means for those accumulators, etc..